It has been possible to ascertain over 30 families with a diagnosis of progressive myoclonic epilepsy with dementia (PME D) in the United States. Examination of affected individuals and their relatives, documentation of family history and review of medical records provided the basis for an initial classification of these disorders. Laboratory studies including analysis of polyglucosan and mucopolysaccharide in various tissue are in progress. On the basis of the examination of thirty-seven affected individuals in nineteen families the following classification of PME D is suggested: Type I(Lafora) Childhood onset, rapid progression, autosomal recessive inheritance, Lafora bodies often present. Type II Childhood onset, slow progression, autosomal recessive inheritance, no Lafora bodies. Type III (Unverricht-Lundborg) Late adolescent onset, autosomal recessive inheritance, moderate or slow progression, occasional Lafora bodies. Type IV (Hartung) Late onset, variable progression, autosomal dominant inheritance. Type V Late adolescent onset, moderate to slow progression, autosomal dominant inheritance, neural hearing loss.